1. Field of the Invention
The present invention relates generally to a power converter circuit and, in particular embodiments, a power conversion circuit topology that can provide low conversion losses, low generated noise and output power controllability.
2. Related Art
Power converter circuits according to embodiments of the present invention are described below. As discussed below, one preferred application of use of a power converter circuit is for starting and controlling one or more cold cathode fluorescent lamps (CCFLs). Such lamps require a high starting voltage (in the order of 700-1000 volts) for a short period of time to ionize the gas contained within the lamp tubes and fire or ignite the lamp. After the gas in a CCFL is ionized and, thus, the lamp is fired, less voltage is needed to keep the lamp on.
CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphoric material coated in the inner wall of the tube, resulting in visible light.
Typical lamp parameters are as follows:
______________________________________ Starting Operating Operating Maximum Voltage voltage Current Current (Vrms) V (Vrms) I (mArms) (mArms) ______________________________________ 750 340 5 20 ______________________________________
Therefore, devices intended to ignite or start up such a tube must be able to generate the starting voltage and be able to supply the operating current at the nominal voltage. A simplification of the process may be obtained by assuming equivalent operating point impedances. Based on this assumption, the approximate impedance of the tube at the operating point is: ##EQU1##
For example, using the above-noted values for typical lamp parameters, the impedance of the tube above is: ##EQU2##
The terminal electrical characteristics of the tubes are highly nonlinear. As discussed above, the tubes require a relatively high voltage to start conducting current. Therefore, until start up, their impedance is very high and the current conduction is negligible. Once ignited however, they exhibit a negative resistance which can make them unstable under some conditions resulting in oscillation. In this regard, a high impedance drive is desirable for a stable operation.
Prior methods of powering CCFLs typically included a ROYER oscillator with an appropriate high voltage transformer. In some cases, when dimming is a requirement, a preregulating buck regulator stage is also included. However, buck regulator waveforms are generated by hard switching, which can result in switching losses and switching noise. In other prior systems, dimming functions are provided by gating the ROYER oscillator on and off to modulate the amount of light emanating from the CCFLs.
FIG. 1 shows a basic ROYER oscillator 10, connected for powering CCFLs 12 and 14. The oscillator 10 has a DC power input 16 coupled to the center terminal of the primary coil of a transformer 18. A primary capacitor 24 is coupled in parallel with the primary coil of transformer 18. One end of the primary coil is coupled to the collector of a first bipolar transistors 20. The other end of the primary coil is coupled to the collector of a second bipolar transistor 22. The emitters of both transistors 20 and 22 are coupled to ground. The collector of transistor 20 is further coupled to the base of transistor 22 through a resistor 26 and the collector of transistor 22 is further coupled to the base of transistor 20 through a resistor 28. In FIG. 1, the pair of parallel coupled CCFLs 12 and 14 are coupled across the secondary coil of transformer 18. Capacitors 30 and 32 represent ballasting capacitors of the CCFLs 12 and 14, respectively. The circuit provides an alternating current power signal to the lamps 12 and 14.
The power conversion function of the circuit shown in FIG. 1 can be relatively inefficient. For example, if a preceding stage (such as a buck regulator preceding the convertor) is used to control the current (e.g., for providing a dimming function), the efficiency losses of each stage becomes a factor in the overall efficiency of the system. Thus, if the current control stage has an 80% efficiency and the FIG. 1 circuit has an 80% efficiency, the overall efficiency of a both stages combined will be 80% of 80%, or 64%. In addition, the use of bipolar transistors as shown in FIG. 1 can adversely effect the efficiency.
FIG. 2a shows a block representation of a typical ROYER oscillator 10' which is gated on and off for providing dimming functions. The oscillator 10' receives a DC input signal across terminals 34 and 36 and a dimming control signal at input 38. A pulse-width modulated control signal, such as the square wave signal 40 shown in FIG. 2b, is provided as a dimming control signal to input 38. In response, the oscillator 10' provides a pulse-width modulated current signal, such as the current signal 42 shown in FIG. 2c, to a CCFL 38. Although the circuit of FIG. 2a provides a single stage conversion, it typically provides other problems that make it undesirable, such as reduced lamp life and low frequency audible noise.
FIGS. 3 and 4 show other implementations of CCFL drivers with dimming capabilities. However, these drivers employ a double stage conversion. In addition, these circuits employ four power switches, which increases the cost and complexity of the circuits.
Prior power conversion systems have typically displayed several disadvantages. For example, as discussed above, double stage power conversion (with typical efficiencies in the 80% range for each stage) can be relatively inefficient. Additional disadvantages of various prior systems include the use of a hybrid of hard switching, resonant mode double stage power conversion devices, unclamped voltages for the power switches (uncoordinated shutdown of power FETs may cause them to destruct due to the discharge of the buck inductor current), the use of four power switches, the use of a front end buck regulator which operates at a different frequency than the ROYER invertor circuit and which may cause frequency beat, restricted dimming range for stable operation, higher standby currents and limited input voltage and power.
Thus, there is a need in the industry for a power conversion circuit topology that can provide low conversion losses, low generated noise and output power controllability. While the prior art systems described above and embodiments of the invention described below relate to power conversion and control circuits for CCFLs, it will be understood that further embodiments of the invention may be employed for any suitable power conversion need.